1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device and more particularly to imaging lenses which are built in image pickup devices mounted in increasingly compact and low-profile mobile terminals such as smart phones, mobile phones and PDAs (Personal Digital Assistants), and game consoles and information terminals such as PCs.
2. Description of the Related Art
In recent years, there has been a tendency that image pickup devices mounted in mobile terminals such as smart phones provide high resolution to cope with an increase in the number of pixels. Also, the imaging lenses built in these image pickup devices are required to provide high resolution, compactness, low-profileness (thinness) and high brightness. In addition, wide-angle cameras capable of capturing an image of an object in a wide range are expected to be used in a wider range of application fields than before. For example, the need for compact monitoring cameras and security cameras has been increasing, and recently robot vacuum cleaners in which a camera controllable remotely by a smart phone is mounted and eyeglass type head-mounted displays with cameras have been developed, rapidly. The imaging lenses of the image pickup devices mounted in these products are strongly expected to provide not only high performance and compactness but also a wider field of view or wider viewing angle than the conventional imaging lenses.
However, if the photographic field of view is particularly widened, a problem may arise that correction of aberrations on the peripheral portion is very difficult and high optical performance is not ensured. Therefore, it is difficult to realize an imaging lens which meets the needs for high resolution, compactness, and low-profileness and also the demand for a wider field of view.
The imaging lenses described in the following Patent Documents 1 to 3 are known as imaging lenses intended to meet the needs for a wider field of view and higher performance.
JP-A-2009-075141 (Patent Document 1) discloses an imaging lens in which elements are arranged in the following order from an object side: a negative first lens having a concave surface on an image side; a positive second lens having a convex surface on the image side; an aperture stop; a third lens as a plano-convex lens having a flat surface on the object side or a biconvex lens having a surface with a large absolute value of curvature radius on the object side; and a cemented lens with positive composite refractive power, composed of a fourth lens and a fifth lens.
JP-A-2010-271541 (Patent Document 2) discloses an imaging lens in which elements are arranged in the following order from an object side to an image side: a first lens as a biconvex lens with positive refractive power; a second lens with negative refractive power having a concave surface on the image side; a third lens as a meniscus lens with positive refractive power having a convex surface on the image side; and a fourth lens as a double-sided aspheric lens with negative refractive power having a concave surface on the image side near an optical axis.
JP-A-2010-026434 (Patent Document 3) discloses an imaging lens intended to be compact and correct various aberrations properly, in which elements are arranged in the following order from an object side: a positive first lens, a positive second lens, a negative third lens, a positive fourth lens, and a negative fifth lens.
The imaging lens described in Patent Document 1 provides high brightness with an F-value of 2.0 and also achieves a wide field of view of about 90 degrees through its first lens with negative refractive power. However, its total track length is about 18 mm, so it cannot meet the needs for compactness and low-profileness. Furthermore, since all the lens surfaces are spherical, it is difficult to correct aberrations and achieve high imaging performance. In fact, the disclosed aberration diagrams indicate that spherical aberrations are serious. Furthermore, since all the lenses are made of glass material, it is difficult to reduce cost.
The imaging lens described in Patent Document 2 has a total track length of about 5.4 mm and the ratio of the total track length to the diagonal length of the effective imaging plane of the image sensor is about 0.9. This suggests that the imaging lens is relatively low-profile. Also, the maximum field of view is in the range from about 70 degrees to 75 degrees, so it provides a relatively wide field of view but it cannot meet the recent demand for a wider field of view. In addition, its F-value is about 2.8, so it cannot be said to provide sufficient brightness to cope with a compact, high-pixel image sensor.
The imaging lens described in Patent Document 3 provides high brightness with an F-value of 2.0 to 2.5 or so and has an ability to correct various aberrations properly. Since it is composed of five constituent lenses, it is advantageous in correcting various aberrations but disadvantageous in terms of compactness and low-profileness. Its total track length is as long as about 7.8 mm and the ratio of total track length to maximum image height is about 1.1. The maximum field of view is about 62 degrees. Therefore, the imaging lens described in Patent Document 3 also does not meet all the above recent needs.
As mentioned above, in the related art, it is difficult to provide the required high resolution, compactness and low-profileness and also meet the demand for a wide field of view.